Re: [PATCH][RFC] Make expansion of balanced binary trees of switches on tree level.

["Martin Liška" <mliska@suse.cz>]

On 08/03/2017 02:52 PM, Steven Bosscher wrote:
> On Wed, Aug 2, 2017 at 1:20 PM, Martin Liška wrote:
>> Hello.
>>
>> After some discussions with Honza, I've decided to convert current code in stmt.c that
>> is responsible for switch expansion. More precisely, I would like to convert the code
>> to expand gswitch statements on tree level. Currently the newly created pass is executed
>> at the end of tree optimizations.
> 
> Hah, something I promissed myself (and others) to do years ago! Thanks
> thanks thanks! :-)

Hi.

Yes, it's very interesting topic to work on. Well, if you have sparse time, help will be
highly appreciated ;)

> 
> The end of the gimple optimizations is seems late for the lowering.
> Before there were gimple optimizations, switch lowering was part of
> the first compiler pass (to generate RTL from the front end) *before*
> all code transformation passes ("optimizations" ;-). Because the
> lowering of switch statements was rewritten as a gimple lowering pass,
> it now runs *after* optimizations. But to be honest, I can't think of
> any optimization opportunities exposed by lowering earlier than at the
> end of gimple optimizations. Years ago there was some RTL jump
> threading still done after lowering of small switch statements, but I
> can't find the related PRs anymore.
> 
> 
>> My plan for future is to inspire in [1] and come up with some more sophisticated switch
>> expansions. For that I've been working on a paper where I'll summarize statistics based
>> on what I've collected in openSUSE distribution with specially instrumented GCC. If I'll be
>> happy I can also fit in to schedule of this year's Cauldron with a talk.
> 
> Sayle's paper is a good starting point. Also interesting:

Yes, I read that.

> 
> http://llvm.org/devmtg/2017-02-04/Efficient-clustering-of-case-statements-for-indirect-branch-prediction.pdf
> About adjusting the size of jump tables to the capabilities of the CPU
> branch predictor. Mixed results but something to consider.

I've also considered to play with jump tables and their sizes. Thanks for the article.

> 
> Kannan & Proebsting, "CORRECTION TO ‘PRODUCING GOOD CODE FOR THE CASE
> STATEMENT’"
> About finding "clusers" of given density within the target values of
> the switch statement. The clustering algorithm as presented is N^2 in
> the number of case labels, but the idea is interesting and a
> simplified, cheaper approach may be possible. This is already used in
> LLVM, it seems.

I'll take a look at LLVM as I'm unable to find PDF of the Kannan's article :/

> 
> The real challenge will be to figure out how to pick from all these
> different approaches the right ones to lower a single switch
> statement.

Exactly, that's why I started with porting of the balanced decision tree to tree level
and I've been collecting statistics. That should help us what would be beneficial and
what's more nice-to-have stuff.

Martin

> 
> Ciao!
> Steven
>